A typical photovoltaic cell includes a p-n junction, which can be formed by a layer of n-type semiconductor in direct contact with a layer of p-type semiconductor. The electronic differences between these two materials create a built-in electric field and potential difference. When a p-type semiconductor is placed in intimate contact with an n-type semiconductor, then a diffusion of electrons can occur from the region of high electron-concentration (the n-type side of the junction) into the region of low electron-concentration (the p-type side of the junction). The diffusion of carriers does not happen indefinitely, however, because of an opposing electric field created by the charge imbalance. The electric field established across the p-n junction induces separation of charge carriers that are created as result of photon absorption. When light is incident on this junction, the photons can be absorbed to excite pairs of electrons and holes, which are “split” by the built-in electric field, creating a current and voltage.
The majority of photovoltaic cells today are made using relatively thick pieces of high-quality silicon (approximately 200 μm) that are doped with p-type and n-type dopants. The large quantities of silicon required, coupled with the high purity requirements, have led to high prices for solar panels. Thin-film photovoltaic cells have been developed as a direct response to the high costs of silicon technology. Thin-film photovoltaic cells typically use a few layers of thin films (≦5 μm) of low-quality polycrystalline materials to mimic the effect seen in a silicon cell. A basic thin-film device consists of a substrate (e.g., glass, metal foil, plastic), a metal-back contact, a 1-5 μm semiconductor layer to absorb the light, another semiconductor layer to create a p-n junction and a transparent top conducting electrode to carry current. Since very small quantities of low-quality material are used, costs of thin-film photovoltaic cells are lower than those for silicon.
The two primary technologies in the thin-film solar space are copper indium gallium sulfur/selenide (CIGS) and cadmium telluride (CdTe). CIGS and CdTe photovoltaic cells have lower costs per watt produced than silicon-based cells and are making significant inroads into the photovoltaic market. However, CIGS and CdTe technologies are likely to be limited by the potential higher costs, lower material availability, and toxicity of some of their constituent elements (e.g., indium, gallium, tellurium, cadmium).